Traumatic knee injuries, such as ligament tears and meniscus tears, degenerative joint diseases, such as arthritis, and overall wear and tear can lead to repairing or replacing a knee joint—referred to as an “arthroplasty” procedure. A conventional arthroplasty procedure involves remodeling, realigning and, in some instances, the replacement of the damaged knee joint with prosthetic implants. For example, in a total knee replacement procedure, a portion of the femur and tibia, where they come together at the knee joint, are removed and replaced with a femoral shell and a mating tibial plate, respectively, that together function like a healthy knee joint.
To assist in arthroplasty procedures, and particularly partial or total knee replacements, a jig may be used to position any one of a number of possible instruments used to drill, cut, and shape or otherwise operate on the damaged knee area. In the particular case of a total knee replacement, a jig may be positioned on the femur to mount a cutting guide that in turn supports a bone saw or other tool to cut (resect) a portion of the distal region of the femur. Similarly, a jig may be positioned on the tibia to mount a cutting guide that in turn supports a bone saw or other tool to resect a portion of the proximal region of the tibia. After the femur and tibia are prepared, the surgeon mounts the femoral shell and tibial plate.
Images of orthopedic joints that are candidates for partial or total replacement are often formed as MRI images, referred to here as “slices,” with each such image being a projection on a two dimensional image forming substrate. Each such MRI image is actually a three dimensional “voxel,” representing a thickness of approximately 2 mm of partial images of cortical bone, cancellous bone cartilage and open space, with each such material having its own range of grey scales in the MRI image. For a full three dimensional representation of an anatomical surface of interest, it is often necessary to obtain tens to hundreds of MRI slices in each of three views (coronal or front view, axial or top view, and sagittal or side view) for a given anatomical component.
Many of the knee replacement procedures presently use what is characterized as “full segmentation” in order to represent a relevant portion of a femur surface or a tibia surface in three dimensions. This approach requires use of a dense, three dimensional grid of points to accurately represent a surface, especially a surface having cusps or sharp corners with very small associated radii of curvature. This approach has several disadvantages, including the following: (1) this approach may be time consuming, often requiring 4-20 hours of intense numerical work to generate and check the accuracy of the grid point coordinates for a single surface; (2) because of the time required to implement this approach for a single surface, use of this approach in mass manufacturing of custom or semi-custom instruments is limited; (3) this approach may introduce geometrical errors, including closing errors; (4) because of the close spacing of grid points, polynomials of high mathematical degree are used, which can introduce undesirable “ripples” in the mathematical surface produced by a full segmentation process; and (5) formation and analysis of a large number of MRI slices is required.
It is with these observations in mind, among others, that various aspects of the present disclosure were conceived.